1. Field of the Invention
The present invention relates to an optical recording medium on which information is recorded, reproduced, and erased using light.
2. Description of the Related Art
Increases in amounts of information, such as image data for multimedia systems, to be processed have brought about a need to increase the density of memories for storing the information. Once stored, this information must also be easily accessible. Optical disks are an example of a high density, easily accessible memory. However, there has been a need to further increase the transmission rate (amount of information transmitted per unitary time) to and from optical disks. Some methods of increasing the transmission rate include increasing the rotational speed of the disk, increasing the modulation frequency, and reducing the access time of an optical head to the optical disk.
Optical recording medium with metal guide layer allows forming tracks on the optical recording medium at a narrower pitch than can be formed in optical recording medium with guide grooves. As shown in FIG. 1(a), the optical recording medium 100 includes a transparent substrate 101; an enhancement layer 102 which is formed on the substrate 101; a guide layer 103; an interference layer 104; a recording layer 105; a protective layer 106; and a reflective layer 107. Recording regions 110 having predetermined width are formed either a coaxial or spiral pattern. Guide regions 111 are provided along the recording regions 110 and are also coaxial or spiral pattern. Guide layers 103 are provided on the enhancement layer 102 at the guide region 111.
The substrate 101 is formed from glass and the guide layer 103 is formed from a metal such as tantalum (Ta). The 10 material of the recording layer 105 is selected in conformity with the type of recording method used for the optical recording medium 100. For example, tellurium (Te) is suitable in the case of forming pits on the recording layer 105; germanium-tellurium-antimony (GeTeSb) is suitable for phase change recording; and terbium iron cobalt (TbFeCo) is suitable for magneto-optical recording. The reflective layer 107 is made from aluminum (Al) or an alloy thereof. The enhancement layer 102, the interference layer 104, and the protective layer 106 are made from a silicon nitride (SiN). Silicon nitrides contain no oxygen and are fairly impermeable to water vapor. Therefore, silicon nitrides are very useful for preventing oxidation of the recording layer 105.
Generally, the thickness T.sub.e of the enhancement layer 102 is determined by the following formula: EQU T.sub.e =.lambda./(8n.sub.1)
wherein .lambda. is the laser wavelength; and
n.sub.1 is refraction rate of the enhancement layer 102. The thickness T.sub.i of the interference layer 104 is determined by the following equation: EQU T.sub.i =.lambda./(8n.sub.2)
wherein n.sub.2 is the refraction rate of the interference layer 104. The protective layer is usually formed to 30 nm, the recording layer 105 to 25 nm, and the guide layer 103 preferably to 20 nm.
The optical recording medium 100 is produced using the following method. First, the enhancement layer 102 is formed from a layer of silicon nitride sputtered on the substrate 101. Next, a layer of tantalum is formed, also using sputtering techniques, on the enhancement layer 102. A predetermined spiral track pattern is etched in the layer of tantalum using well-known lithography techniques to produce the guide layer 103. In this example, the layer of tantalum is etched using plasma etching, which uses a mixture of flon 14 (CF.sub.4) and oxygen (O.sub.2). In standard etching, changes in etching conditions can cause changes in etching speed. Therefore to insure that the tantalum guide layers are completely formed, the tantalum layer is overetched, that is, etched for a duration of time several tens of percent longer than the etching time calculated as required to produce guide layers from a standard tantalum film at a standard etching speed. Recording region 110 is formed during removal of the guide layer 103. Finally, the SiN interference layer 104, the recording layer 105, the SiN protective layer 106, and the Al reflective layer 107 are formed by sputtering techniques.
Recording is performed in the optical recording medium 100 by passing laser light, collected by, for example, an objective lens (not shown), through recording region 110 and irradiating the recording layer 105 therewith. The irradiated portion of the recording layer 105 is deformed, phase changed, or inversely magnetized, depending on the recording method used, so as to form recording marks. The recording marks have a Kerr rotation angle or a reflectivity that is different from those of non-irradiated portions. The thus-recorded information is reproduced by detecting the changes in Kerr rotation angle or reflectivity at the recording marks.
Tracking is performed using well-known pushpull methods based on light reflected from the guide region 111 and diffracted at the interface between guide region 111 and the recording region 110. Interference light, resulting from interference between the reflected light and the deflected light, is detected at two light receiving portions. Tracking error is determined by detecting difference between intensity of light detected at the two light receiving portions.
Silicon nitride is also etchable using plasma etching, which uses a mixture of CF.sub.4 and O.sub.2. Silicon nitride is etched at a rate of 60 nm every minute compared to a rate of about 120 nm every minute for tantalum. Therefore, after the tantalum layer is completely etched away when forming the guide layers 103, the silicon nitride layer of the enhancement layer 102 is also etched at recording regions 110 as shown in FIG. 1(a). Sputter accumulates at exposed surfaces, that is, at recording regions 110, of the enhancement layer 102 to form rough surfaces in recording regions 110 of the enhancement layer 102. The rough areas in the surface of the enhancement layer 102 translate into rough areas at the recording regions 110 of the interference layer 104, the recording layer 105, the protective layer 106, and the reflective layer 107. The rough areas at each interface become a source of noise when information is recorded and reproduced at the recording region 110 of the recording layer 105. When noise increases, the signal to noise ratio drops, the error rate increases, and reliability also decreases.
Further, when the track pitch is narrowed as a result of providing tantalum guide layers 103, tracking characteristics such as divided pushpull signal and the strength of the pushpull signal drop so that tracking drive can not be stably operated.
When formed to a thickness of 20 nm, a tantalum guide layer 103 greatly effects reflectivity from multiple interference between the reflective layer 107, the guide layer 103, and the recording layer 105. Therefore, as shown in FIG. 1(b) the pushpull signal greatly fluctuates with variation in the thickness of the guide layer 103. When the pushpull signal is at a low point in fluctuations, tracking gain is insufficient for stable tracking. On the other hand, when, also as a result of fluctuations, the pushpull signal is too strong, gain can become excessive so that the servo circuit oscillates, thereby causing tracking error.